I go back and forth on things like lol. It's hard to convey in writing when something should be take lightly or as a joke. Without the standard facial cues of smiling it's easy to take something the wrong way.

And in general your heuristic is correct, but the gym is an odd place. I'm pretty social, but would prefer no one talk to me while working out. I'm usually so busy watching the clock and focusing on the task at hand that I don't want to seem rude to someone by having to eventually ignore them to finish my workout.

This is why we have these punctuation marks called "smilies". :-)

(I fear that all the image-substitution of smilies will create a generation of internet users that can't read the original ascii ones...)